Add support in Nuget package for being referenced as a transitive build dependency

This release implements all exposed rounded corners features (circle, per-quadrant, percentages, pixel radius) with optimized anti-aliasing.

• Add basic support for rounding corners. Per-quadrant and circle rounding is not supported yet. Anti-aliasing is not supported yet
• This also updates the CI process so binaries are built against Ubuntu 18.04 instead of 16.04.

• Fixes #582 - Corrupted jpegs are now handled gracefully.

• Optimize PNG and WebP filesize when converting from WebP by decoding without alpha when appropriate
• Fix compositing when the canvas is a decoded PNG (Fixes #580)

We have replaced Imageflow.NativeRuntime.osx_10_11-x86_64 with Imageflow.NativeRuntime.osx-x86_64 and Imageflow.NativeTool.osx_10_11-x86_64 with Imageflow.NativeTool.osx-x86_64 and deprecated the older packages.

The new packages require macOS 10.15 or higher to run.

• Fix trim.threshold command string - was causing ContentDependent LayoutError

• Make LibPngEncoder{matte} work finally
• Optimize png size when applying opaque matte color
• Default to white background when converting PNG32 to jpeg
• Add matte support to LibjpegTurbo and MozJpeg encoder presets. Fixes #417
• Make Constraint.canvas_color override hints.background_color when set.